1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving method to be used in the liquid crystal display device and more particularly to the liquid crystal display device suitably used when either of transmissive display or reflective display is selected depending on variations in environments for using the liquid crystal display device and the driving method to be used in the liquid crystal display device.
The present application claims priority of Japanese Patent Application No. 2005-044855 filed on Feb. 21, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
A liquid crystal display device is classified into two types of devices, one being a transmissive-type liquid crystal display device in which light emitted from an embedded backlight is transmitted through a liquid crystal panel to achieve displaying and another being a reflective-type liquid crystal display device in which external light such as sunlight is reflected in a liquid crystal panel to achieve displaying. In the liquid crystal display device, to perform a color display operation, ordinarily, a color filter is used. However, in recent years, a new type of liquid crystal device is fabricated which performs a field sequential (FS) driving operation by using a light source made up of three kinds of light emitting elements each emitting colored light corresponding to one of three primary colors of red (R), green (G), and blue (B). In the FS driving operation, as a backlight, for example, RGB three-primary color lamps, one emitting R, another emitting G, and another emitting B are used and each of the lamps is turned ON or OFF in a timeshared manner.
The conventional liquid crystal display device of this type includes, as shown in FIG. 17, an upper-side substrate 1, a lower-side substrate 2, an upper-side retardation plate 3, an upper-side polarizer 4, a lower-side retardation plate 5, and a lower-side polarizer 6. On the upper-side substrate 1 are formed a plurality of data electrodes 7, a plurality of scanning electrodes 8, a plurality of TFTs (Thin Film Transistors) 9, and a plurality of pixel electrodes 10. The data electrodes 7 are formed in a first direction at predetermined intervals to input a sub-pixel data signal corresponding to each sub-pixel obtained by dividing each pixel into three portions each emitting one of the three primary colors of R, G, and B. The scanning electrodes 8 are formed in a second direction orthogonal to the first direction to input a scanning signal. The TFTs 9 and pixel electrodes 10 make up pixel cells formed in a region of intersections of each of the data electrodes 7 and each of the scanning electrodes 8. On the lower-side substrate 2 are formed a reflecting plate/coloring layer 11 and a coloring layer 12. The coloring layer 11 is made up of the three primary color layers of R, G, and B being adjacent to one another. On a side opposite to visually-recognized side (viewer side) of the lower polarizer 6 is provided a light source (backlight). The backlight is made up of three kinds of light emitting elements each emitting light corresponding to one of the three primary colors of R, G, and B.
FIG. 18 is a cross-sectional view of the liquid crystal panel of FIG. 17 taken along a line A-A. As shown in FIG. 18, a liquid crystal layer 14 is sandwiched between the upper-side substrate I and the lower-side substrate 2 and a semi-transmissive light reflecting film (hereinafter referred to as semi-transmissive reflecting film) 13 is formed on the liquid crystal layer 14 side on the lower-side substrate 2. One coloring layer 12 is formed on all regions (aperture portion) of each pixel cell.
In the conventional liquid crystal display device, by supplying a sub-pixel data signal to a pixel cell selected by a scanning signal, out of pixel cells formed in a region of intersections of the data electrodes 7 and scanning electrodes 8, light fed from a viewer side or the light source is modulated in a manner to correspond to the display screen. In this case, the FS driving operation is performed, that is, a color display operation is performed according to a successive additive color mixture method in which one frame is divided into three fields and each of the light emitting elements including one emitting R light, another emitting G light, and another emitting B light is turned ON in every field sequentially in terms of time. At time of the transmissive display operation, by simultaneous lighting of the light emitting elements including one emitting R light, another emitting G light, and another emitting B light, white light is provided and a color display operation is performed according to a juxtapositional additive color mixture method by using light passing through the coloring layer 12. At time of the reflective display operation, by passing of external light such as sunlight through the reflecting plate/coloring layer 11 and reflection of the light, the color display operation is performed according to the juxtapositional additive color mixture method.
A conventional technology of this type, in addition to the above liquid crystal display device, is disclosed in a following reference. That is, a liquid crystal display device disclosed in Reference 1 (Japanese Patent Application Laid-open No. 2003-076342, Abstract, FIG. 1) includes a liquid crystal display panel having a color filter and a backlight. The color filter has a plurality of kinds of coloring layers different in color. The backlight has a plurality of types of light emitting elements each emitting colored light corresponding to a color of each coloring layer. In the case of the transmissive display operation, display is performed by the field sequential driving operation. That is, one frame is divided into a plurality of fields and, during one frame, a plurality of types of light emitting elements emits light sequentially in terms of time and the liquid crystal panel is driven in every field in synchronization with light emitting timing of the light emitting elements.
However, the above conventional liquid crystal display device has following problems. The first problem is that, not only mounting of the upper-side retardation plate 3 and lower-side retardation plate 5 but also a process of forming the reflecting plate/coloring layer 11 is required, which causes the number of components and fabricating processes to be increased and makes it difficult to provide a low-priced product. The second problem is that, at the time of the transmissive display operation, a display screen is dark. The third problem is that three kinds of data signals to correspond to the FS driving, transmissive display, and reflective display are necessary, which causes configurations to be complicated.
The reason for the occurrence of the first problem is that precise patterning on the reflecting plate/coloring layer 11 is required, which causes fabricating processes to increase and yields to be reduced. The fabricating process thus includes a series of complicated processes including stacking of the reflecting plate in layers, coating of a resist, exposure of patterns, development, etching on the reflecting plate, peeling of a resist, and cleaning. Moreover, in the case of using a mirror reflecting plate as the reflecting plate, a dark display screen appears when the display is seen from a direction other than a direction of specular reflection of incident light, which makes it difficult to see the display fully in some cases. The light reflected specularly is superimposed on surface reflected light fed from the upper-side polarizer 4 and lower-side polarizer 6 and, as a result, its viewability decreases. To solve this problem, formation of bumps and dips to reduce the specularly reflected light component by using a resin or a like or insertion of a diffusive sheet in which tiny transparent particles are dispersed between the upper-side polarizer 4 and lower-side polarizer 6 and between the upper-side substrate 1 and lower-side substrate 2 is required, which increases further the number of fabricating processes and decreases cost-performance. Moreover, it is necessary that, on a side being opposite to the liquid crystal layer 14 on the upper-side substrate 1 and lower-side substrate 2 are stacked the upper-side retardation plate 3 and lower-side retardation plate 5 and the upper-side polarizer 4 and lower-side polarizer 6 in layers, which also decreases yields. In particular, as a material for the upper-side retardation plate 3 and lower-side retardation plate 5, a norbornene transparent resin having a lower wavelength dispersion characteristic is used, however, the material is expensive, which makes it difficult to lower the price of the liquid crystal display device.
The reason for the occurrence of the second problem is that, as shown in FIG. 18, the coloring layer 12 is formed in an entire aperture of each pixel cell. As a result, the coloring layer 12 is formed in both a transmissive region and a reflective region of each pixel cell and light fed from the backlight is absorbed by the coloring layer 12. For example, when red is to be displayed, the pixel cell corresponding to blue and green coloring layers 12 are turned OFF and no light is emitted from the pixel cell, however, light passes through the blue and green coloring layer 12 and, therefore, efficiency of using light is decreased when compared with the case of the FS driving operation in which only the transmissive driving operation is performed without using the coloring layer 12. As shown in FIG. 19, when the coloring layer 12 is used, a ratio of the coloring layer 12 to the transmissive aperture is 100%. and a ratio of luminance occurring when the coloring layer is used to luminance occurring when the FS driving operation is performed without using the coloring layer is about 55% in the case (1) when pixel cells in a pixel are driven simultaneously and about 30% in the case (2) when pixel cells in the pixel are driven individually. In both the cases, luminance is lowered accordingly.
Therefore, in order to obtain luminance being equivalent to luminance obtained by the FS driving operation in which only the transmissive driving operation is performed without using the coloring layer 12, the luminance needs to be higher at least two times to three times than the luminance of the backlight and, as a result, a problem arises that power consumption is greatly increased. Moreover, at the time of the reflective display operation, light passes through the coloring layer 12 twice, whereas, at the time of the transmissive display operation, light passes through the coloring layer 12 only once. Therefore, if the coloring layer 12 is so configured as to operate optimally in the reflective display, color reproducibility decreases in the transmissive display and if the coloring layer 12 is so configured as to operate optimally in the transmissive display, a display screen becomes dark in the reflective display. For example, when a chromaticity region being equivalent to 40% of the region specified by NTSC (National Television System Committee) is to be obtained at the time of the reflective display operation, the chromaticity region becomes about 20% of the region specified by NTSC at the time of the transmissive display. Also, if a chromaticity region being equivalent to 40% of the region specified by NTSC is to be obtained at the time of the transmissive display, external light is almost absorbed and a dark display appears in the reflective display.
The reason for the occurrence of the third problem is that, since a driving frequency of the data signal differs depending on the FS driving, transmissive display, reflective display, a circuit to generate data signal corresponding to each of the FS driving, transmissive display, and reflective display operations needs to be provided. Moreover, in the case of the FS driving, the pixel cells each making up one pixel includes a pixel cell which is turned ON for displaying and a pixel cell which is turned OFF for no displaying and, therefore, another additional signal besides the data signal needs to be generated. That is, a signal to drive the R coloring layer 12, a signal to drive the G coloring layer 12, and a signal to drive the B coloring layer 12 need to be individually generated and, therefore, the number of data signals being larger by three times than that required when each pixel cell in a pixel is simultaneously driven. Also, even when each pixel cell is driven simultaneously, a signal for each pixel cell needs to be generated, which causes the configurations of the liquid crystal display device to be more complicated.
In addition, as shown in FIG. 17 or FIG. 18, the TFT 9 are formed on the upper-side substrate 2. However, since it is impossible to form the TFTs 9 directly on a thin-film substrate or plastic, after formation of the TFTs 9 on a glass substrate with a thickness of about 0.5 mm to 1.0 mm, a chemical etching method or physical polishing method is performed on the TFT 9 to obtain their thin configuration. In this case, control of a film thickness is difficult and there is a limit to a degree (for example, 0.3 mm) to which a film is made thin. Due to this, a problem arises that a decrease in visual recognition caused by the occurrence of parallax and/or color mixture occurs. Moreover, specific gravity of a glass substrate is comparatively high and it is, therefore, difficult to make the liquid crystal display device lightweight and thin to a degree to which a mobile device such as a portable phone or PDA (Personal Digital Assistants) requires.
Furthermore, the liquid crystal display device disclosed in the above Patent Reference 1, since it has configurations similar to those of the liquid crystal display device shown in FIG. 17, has the same problems as described above.